Structures and Unions in C: From Basics to Advanced

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Last Updated on: 24th Jul 2025 19:31:27 PM

Structures and unions in C are like custom toolkits, allowing you to bundle related data or share memory efficiently to model real-world entities. Structures group different data types into a single unit, while unions provide a memory-efficient way to store one of several types. This tutorial covers Defining and Using Structures, Nested Structures, Array of Structures, and Unions and Differences from Structures, progressing from beginner-friendly concepts to advanced, real-life applications. Designed for your website, it features unique, engaging examples with user input via scanf and fgets to captivate students. Each example includes code, output, and a detailed explanation to ensure clarity and excitement. Let’s explore structures and unions!

 

1. Introduction to Structures and Unions

 

1.1. Structures

A structure in C is a user-defined data type that groups variables of different types into a single unit, like a blueprint for an object (e.g., a student record with name, ID, and grade).

 

Key Features:

• Defined using the struct keyword.

• Members accessed using the dot (.) or arrow (->) operator (for pointers).

• Ideal for modeling complex entities like records or objects.

 

1.2. Unions

A union is a user-defined type where all members share the same memory location, allowing only one member to hold a value at a time, like a multi-purpose container.

 

Key Features:

• Defined using the union keyword.

• Memory-efficient, as the size is determined by the largest member.

• Useful for type punning or saving space when only one member is needed.

 

Why Use Structures and Unions?

• Structures: Organize related data (e.g., employee details).

• Unions: Optimize memory for mutually exclusive data (e.g., variant types).

• Both: Enable real-world modeling in applications like databases or systems.

 

2. Defining and Using Structures

 

Syntax:

struct structure_name {
    data_type member1;
    data_type member2;
    // ...
};
struct structure_name variable; // Declare variable

 

Accessing Members:

• Dot operator: variable.member

• Arrow operator (for pointers): ptr->member

 

Basic Example: Student Record

This program defines a structure for a student and takes user input to store and display details.

 

#include <stdio.h>
#include <string.h>
struct Student {
    char name[50];
    int id;
    float grade;
};
int main() {
    struct Student student;
    
    printf("Enter student name: ");
    fgets(student.name, 50, stdin);
    student.name[strcspn(student.name, "\n")] = '\0';
    
    printf("Enter student ID: ");
    scanf("%d", &student.id);
    
    printf("Enter student grade (0-100): ");
    scanf("%f", &student.grade);
    
    printf("\nStudent Details:\n");
    printf("Name: %s\n", student.name);
    printf("ID: %d\n", student.id);
    printf("Grade: %.2f\n", student.grade);
    return 0;
}

 

Output (example user input: Alice Smith, 101, 85.5):

Enter student name: Alice Smith
Enter student ID: 101
Enter student grade (0-100): 85.5

Student Details:
Name: Alice Smith
ID: 101
Grade: 85.5

 

Explanation:

• The Student structure groups a name (char array), ID (int), and grade (float).

• fgets reads the name with spaces, and strcspn removes the newline.

• scanf inputs the ID and grade.

• The dot operator (student.name, etc.) accesses members to display the data.

 

3. Nested Structures

A nested structure is a structure containing another structure as a member, allowing hierarchical data organization (e.g., a student with an address).

 

Syntax:

struct Outer {
    data_type member1;
    struct Inner {
        data_type member2;
    } inner;
};

 

Example: Employee with Address

This program defines a nested structure for an employee with an address and takes user input.

#include <stdio.h>
#include <string.h>
struct Address {
    char city[50];
    int zip;
};
struct Employee {
    char name[50];
    int emp_id;
    struct Address address;
};
int main() {
    struct Employee emp;
    
    printf("Enter employee name: ");
    fgets(emp.name, 50, stdin);
    emp.name[strcspn(emp.name, "\n")] = '\0';
    
    printf("Enter employee ID: ");
    scanf("%d", &emp.emp_id);
    
    printf("Enter city: ");
    scanf(" %[^\n]", emp.address.city); // Reads string with spaces
    printf("Enter zip code: ");
    scanf("%d", &emp.address.zip);
    
    printf("\nEmployee Details:\n");
    printf("Name: %s\n", emp.name);
    printf("ID: %d\n", emp.emp_id);
    printf("City: %s\n", emp.address.city);
    printf("Zip: %d\n", emp.address.zip);
    return 0;
}

 

Output (example user input: John Doe, 502, New York, 10001):

Enter employee name: John Doe
Enter employee ID: 502
Enter city: New York
Enter zip code: 10001

Employee Details:
Name: John Doe
ID: 502
City: New York
Zip: 10001

 

Explanation:

• The Address structure is nested inside the Employee structure.

• fgets and scanf input the employee’s name, ID, city, and zip code.

• The nested structure is accessed using emp.address.city and emp.address.zip.

• The program organizes hierarchical data (employee → address) clearly.

 

4. Array of Structures

An array of structures stores multiple structure instances, like a list of records (e.g., multiple students).

 

Syntax:

struct structure_name array[size];

 

Real-Life Example 1: List of Employees

Suppose you are making a system to manage 3 employees in a company. Each employee has:

• empId

• name

• salary

 

We can define a structure Employee and then create an array to hold all 3 employees.

#include <stdio.h>

// Defining structure
struct Employee {
    int empId;
    char empName[50];
    float salary;
};

int main() {
    // Array of 3 employees
    struct Employee emp[3];

    // Input details
    for(int i = 0; i < 3; i++) {
        printf("Enter ID, Name, and Salary for Employee %d:\n", i+1);
        printf("ID: ");
        scanf("%d", &emp[i].empId);

        printf("Name: ");
        scanf(" %[^\n]", emp[i].empName); // Reads string with spaces

        printf("Salary: ");
        scanf("%f", &emp[i].salary);
        printf("\n");
    }

    // Display details
    printf("\n--- Employee Details ---\n");
    for(int i = 0; i < 3; i++) {
        printf("Employee %d: ID = %d, Name = %s, Salary = %.2f\n", 
               i+1, emp[i].empId, emp[i].empName, emp[i].salary);
    }

    return 0;
}

 

 

Enter ID, Name, and Salary for Employee 1:
ID: 101
Name: Ravi Kumar
Salary: 45000

 

Enter ID, Name, and Salary for Employee 2:
ID: 102
Name: Meena Singh
Salary: 47000

 

Enter ID, Name, and Salary for Employee 3:
ID: 103
Name: Anil Sharma
Salary: 43000

 

--- Employee Details ---
Employee 1: ID = 101, Name = Ravi Kumar, Salary = 45000.00
Employee 2: ID = 102, Name = Meena Singh, Salary = 47000.00
Employee 3: ID = 103, Name = Anil Sharma, Salary = 43000.00
 

Explanation:

• struct Employee emp[3]; declares an array to store 3 employees.

• scanf(" %[^\n]", emp[i].empName); is used to read names with spaces.

• The loop inputs and then prints each employee's data.

• This is a realistic structure used in HR systems, payroll, etc.

 

Example 2: Classroom Gradebook

This program manages a classroom of 3 students, taking input and calculating the average grade.

#include <stdio.h>
#include <string.h>
#define SIZE 3
struct Student {
    char name[50];
    float grade;
};
int main() {
    struct Student students[SIZE];
    float total = 0.0;
    
    printf("Enter details for %d students:\n", SIZE);
    for (int i = 0; i < SIZE; i++) {
        printf("\nStudent %d:\n", i + 1);
        printf("Name: ");
        fgets(students[i].name, 50, stdin);
        students[i].name[strcspn(students[i].name, "\n")] = '\0';
        
        printf("Grade (0-100): ");
        scanf("%f", &students[i].grade);
        getchar(); // Clear newline
        total += students[i].grade;
    }
    
    printf("\nClassroom Gradebook:\n");
    for (int i = 0; i < SIZE; i++) {
        printf("Student %d: %s, Grade: %.2f\n", i + 1, students[i].name, students[i].grade);
    }
    printf("Average Grade: %.2f\n", total / SIZE);
    return 0;
}

 

Output (example user input: Alice 90, Bob 85, Charlie 88):

Enter details for 3 students:

Student 1:
Name: Alice
Grade (0-100): 90

Student 2:
Name: Bob
Grade (0-100): 85

Student 3:
Name: Charlie
Grade (0-100): 88

Classroom Gradebook:
Student 1: Alice, Grade: 90.00
Student 2: Bob, Grade: 85.00
Student 3: Charlie, Grade: 88.00
Average Grade: 87.67

 

Explanation:

• An array of Student structures stores data for 3 students.

• fgets and scanf input each student’s name and grade, with getchar() clearing the newline.

• A loop calculates the total grade, and the average is computed.

• The program displays all students and their average, modeling a simple gradebook.

 

When to Use Array of Structures?

• When you have multiple records of the same entity (students, books, cars).

• Useful in search, sort, update based on specific fields (like highest salary).

 

5. Unions and Differences from Structures

 

5.1. Unions

A union allows multiple members to share the same memory location, with only one member active at a time. The size of a union is the size of its largest member.

 

Syntax:

union union_name {
    data_type member1;
    data_type member2;
    // ...
};
union union_name variable;

 

 

Differences from Structures:

• Memory: Structures allocate memory for all members; unions allocate memory for the largest member only.

• Usage: Structures store all members simultaneously; unions store one member at a time.

• Size: sizeof(struct) is the sum of member sizes (plus padding); sizeof(union) is the size of the largest member.

 

Example: Product Variant Storage

This program uses a union to store either a book’s page count or a gadget’s battery capacity, demonstrating memory sharing.

#include <stdio.h>
#include <string.h>
union ProductData {
    int pages; // For books
    float battery; // For gadgets
};
struct Product {
    char name[50];
    char type[20]; // "book" or "gadget"
    union ProductData data;
};
int main() {
    struct Product product;
    
    printf("Enter product name: ");
    fgets(product.name, 50, stdin);
    product.name[strcspn(product.name, "\n")] = '\0';
    
    printf("Enter product type (book/gadget): ");
    scanf("%s", product.type);
    
    if (strcmp(product.type, "book") == 0) {
        printf("Enter page count: ");
        scanf("%d", &product.data.pages);
    } else if (strcmp(product.type, "gadget") == 0) {
        printf("Enter battery capacity (mAh): ");
        scanf("%f", &product.data.battery);
    } else {
        printf("Invalid type!\n");
        return 1;
    }
    
    printf("\nProduct Details:\n");
    printf("Name: %s\n", product.name);
    printf("Type: %s\n", product.type);
    if (strcmp(product.type, "book") == 0) {
        printf("Pages: %d\n", product.data.pages);
    } else {
        printf("Battery: %.2f mAh\n", product.data.battery);
    }
    printf("Size of union: %zu bytes\n", sizeof(product.data));
    return 0;
}

 

Output (example user input: Kindle, book, 300):

Enter product name: Kindle
Enter product type (book/gadget): book
Enter page count: 300

Product Details:
Name: Kindle
Type: book
Pages: 300
Size of union: 4 bytes

 

Output (example user input: Smartwatch, gadget, 400.5):

Enter product name: Smartwatch
Enter product type (book/gadget): gadget
Enter battery capacity (mAh): 400.5

Product Details:
Name: Smartwatch
Type: gadget
Battery: 400.50 mAh
Size of union: 4 bytes

 

Explanation:

• The ProductData union stores either pages (int) or battery (float), sharing the same 4-byte memory.

• The Product structure includes the union and a type field to track the active member.

• Based on type, the program inputs either pages or battery using scanf.

• The union’s size is 4 bytes (largest member), saving memory compared to a structure.

 

6. Advanced Example: Library Management System

This real-life program simulates a library system, using nested structures, an array of structures, and a union to manage books and e-books, including user input for book details and search functionality.

 

#include <stdio.h>
#include <string.h>
#define MAX_BOOKS 3
union BookFormat {
    int pages; // For physical books
    float file_size; // For e-books (MB)
};
struct Publication {
    int year;
    char publisher[50];
};
struct Book {
    char title[50];
    char author[50];
    char format[20]; // "physical" or "ebook"
    union BookFormat details;
    struct Publication pub;
};
void inputBook(struct Book *book) {
    printf("Enter book title: ");
    fgets(book->title, 50, stdin);
    book->title[strcspn(book->title, "\n")] = '\0';
    
    printf("Enter author: ");
    fgets(book->author, 50, stdin);
    book->author[strcspn(book->author, "\n")] = '\0';
    
    printf("Enter format (physical/ebook): ");
    scanf("%s", book->format);
    
    if (strcmp(book->format, "physical") == 0) {
        printf("Enter page count: ");
        scanf("%d", &book->details.pages);
    } else if (strcmp(book->format, "ebook") == 0) {
        printf("Enter file size (MB): ");
        scanf("%f", &book->details.file_size);
    }
    
    printf("Enter publication year: ");
    scanf("%d", &book->pub.year);
    getchar(); // Clear newline
    
    printf("Enter publisher: ");
    fgets(book->pub.publisher, 50, stdin);
    book->pub.publisher[strcspn(book->pub.publisher, "\n")] = '\0';
}
void searchBook(struct Book books[], int n, char *searchTitle) {
    int found = 0;
    for (int i = 0; i < n; i++) {
        if (strstr(books[i].title, searchTitle)) {
            printf("\nBook Found:\n");
            printf("Title: %s\n", books[i].title);
            printf("Author: %s\n", books[i].author);
            printf("Format: %s\n", books[i].format);
            if (strcmp(books[i].format, "physical") == 0) {
                printf("Pages: %d\n", books[i].details.pages);
            } else {
                printf("File Size: %.2f MB\n", books[i].details.file_size);
            }
            printf("Year: %d\n", books[i].pub.year);
            printf("Publisher: %s\n", books[i].pub.publisher);
            found = 1;
        }
    }
    if (!found) {
        printf("\nNo book found with title containing '%s'.\n", searchTitle);
    }
}
int main() {
    struct Book library[MAX_BOOKS];
    
    printf("Enter details for %d books:\n", MAX_BOOKS);
    for (int i = 0; i < MAX_BOOKS; i++) {
        printf("\nBook %d:\n", i + 1);
        inputBook(&library[i]);
        getchar(); // Clear newline
    }
    
    printf("\nLibrary Catalog:\n");
    for (int i = 0; i < MAX_BOOKS; i++) {
        printf("\nBook %d:\n", i + 1);
        printf("Title: %s\n", library[i].title);
        printf("Author: %s\n", library[i].author);
        printf("Format: %s\n", library[i].format);
        if (strcmp(library[i].format, "physical") == 0) {
            printf("Pages: %d\n", library[i].details.pages);
        } else {
            printf("File Size: %.2f MB\n", library[i].details.file_size);
        }
        printf("Year: %d\n", library[i].pub.year);
        printf("Publisher: %s\n", library[i].pub.publisher);
    }
    
    char searchTitle[50];
    printf("\nEnter title to search: ");
    fgets(searchTitle, 50, stdin);
    searchTitle[strcspn(searchTitle, "\n")] = '\0';
    searchBook(library, MAX_BOOKS, searchTitle);
    
    return 0;
}

 

Output (example user input: {C Programming, K&R, physical, 200, 1978, Prentice}, {Python Guide, Guido, ebook, 2.5, 2000, OReilly}, {C++ Primer, Lippman, physical, 500, 2012, Addison}, search: C):

Enter details for 3 books:

Book 1:
Enter book title: C Programming
Enter author: K&R
Enter format (physical/ebook): physical
Enter page count: 200
Enter publication year: 1978
Enter publisher: Prentice

Book 2:
Enter book title: Python Guide
Enter author: Guido
Enter format (physical/ebook): ebook
Enter file size (MB): 2.5
Enter publication year: 2000
Enter publisher: OReilly

Book 3:
Enter book title: C++ Primer
Enter author: Lippman
Enter format (physical/ebook): physical
Enter page count: 500
Enter publication year: 2012
Enter publisher: Addison

Library Catalog:

Book 1:
Title: C Programming
Author: K&R
Format: physical
Pages: 200
Year: 1978
Publisher: Prentice

Book 2:
Title: Python Guide
Author: Guido
Format: ebook
File Size: 2.50 MB
Year: 2000
Publisher: OReilly

Book 3:
Title: C++ Primer
Author: Lippman
Format: physical
Pages: 500
Year: 2012
Publisher: Addison

Enter title to search: C

Book Found:
Title: C Programming
Author: K&R
Format: physical
Pages: 200
Year: 1978
Publisher: Prentice

Book Found:
Title: C++ Primer
Author: Lippman
Format: physical
Pages: 500
Year: 2012
Publisher: Addison

 

Explanation:

• The BookFormat union stores either pages or file_size, saving memory.

• The Publication structure is nested in the Book structure for publication details.

• An array of Book structures (library) stores 3 books.

• The inputBook function uses a pointer to input data for a single book, handling both physical and e-book formats.

• The searchBook function searches for books by title substring using strstr.

• The program models a real-life library system, demonstrating nested structures, arrays of structures, and unions in a practical context.

 

7. Why Structures and Unions Are Essential

Structures and unions in C are vital for:

• Data Organization: Structures group related data for complex entities (e.g., books).

• Memory Efficiency: Unions optimize storage for mutually exclusive data.

• Real-World Applications: Used in databases, file systems, and simulations.

 

Pro Tip: Experiment with these examples! Try extending the library system to sort books by year, add a function to count e-books, or create a union for different payment types. Structures and unions are your foundation for structured programming!

Happy coding 

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